1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer and an electrophotographic copying machine.
2. Related Background Art
FIG. 13 shows a development bias circuit and a surface potential measurement circuit as a configuration example of an image producing (image forming) control circuit in the image forming apparatus such as the electrophotographic printer and the electrophotographic copying machine. At this point, the conventional development bias circuit will be described as an example of bias generation circuits. Because a constant-voltage system bias generation circuit such as grid bias has the same configuration and control method, the description of the constant-voltage system bias generation circuit is omitted.
In FIG. 13, the reference numeral 11a denotes a photoconductor drum which is rotated in the direction of arrow R1, the reference numeral 12a denotes a primary charger which evenly charges a surface of the photoconductor drum 11a, the reference numeral 18a denotes a surface potential sensor which detects a surface potential at the photoconductor drum 11a, and the reference numeral 14a denotes a development device which develops an electrostatic latent image on the photoconductor drum 11a. 
The reference numeral 70a shows the configuration of the development bias circuit. The development bias circuit 70a has a direct-current bias generation portion 71a, a generation bias detection portion 72a, and a direct-current bias control portion 73a. The reference numeral 90a shows the configuration of the surface potential measurement circuit. The surface potential measurement circuit 90a has a sensor control portion 91a, a sensor direct-current bias generation portion 92a, a sensor generation bias detection portion 93a, and a detection-signal transmission portion 94a. The reference numeral 95 shows an apparatus control portion which controls the image forming apparatus. The apparatus control portion 95 has a D/A conversion portion 96a whose output portion is connected to the development bias circuit 70a and an A/D conversion portion 97a whose output portion is connected to the surface potential measurement circuit 90a. 
In the image producing control circuit having the above configuration, the development bias circuit 70a is operated according to a control signal from the apparatus control portion 95. At first the apparatus control portion 95 directs the development bias circuit 70a to output a desired bias output value by an analog signal level through the D/A conversion portion 96a. In the development bias circuit 70a, the direct-current bias control portion 73a receives the analog signal. In response to the signal from the D/A conversion portion 96a, the direct-current bias control portion 73a operates direct-current bias generation portion 71a to cause the direct-current bias generation portion 71a to generate a direct-current bias which is of a development bias. The direct-current bias generated in the above way is converted into a detection signal by the generation bias detection portion 72a, and the detection signal is transmitted to the direct-current bias control portion 73a. The direct-current bias control portion 73a compares the detection signal to the analog signal from the D/A conversion portion 96a, and the direct-current bias control portion 73a transmits the control signal to the direct-current bias generation portion 71a so that the detection signal and the analog signal agree with each other.
Then, the surface potential measurement circuit 90a is also controlled by the apparatus control portion 95. The sensor control portion 91a transmits a drive signal to the surface potential sensor 18a. The surface potential sensor 18a is operated according to the drive sensor to send out a measurement signal following the potential difference between the surface potential sensor 18a and the photoconductor drum 11a. The sensor control portion 91a receives the signal to operate the sensor direct-current bias generation portion 92a so that the signal is minimized, i.e. the surface potential at the photoconductor drum 11a becomes equal to the potential at the surface potential sensor 18a. 
Thus, the surface potential at the photoconductor drum 11a and the generation bias value of the sensor direct-current bias generation portion 92a is controlled so as to become the same potential. On the other hand, the sensor generation bias detection portion 94a converts the generation bias of the sensor direct-current bias generation portion 92a into the detection signal to transmit the detection signal to the A/D conversion portion 97a through the detection signal transmission portion 94a. The A/D conversion portion 97a performs digital conversion of the detection signal to notify the apparatus control portion 95 of the detection result.
With reference to a technique of improving detection accuracy of the surface potential sensor, Japanese Patent Application Laid-Open No. H08-201461 discloses a method in which switch means for switching the photoconductor drum to a floating state is provided, a reference voltage is provided to the photoconductor drum in the floating state, and detection properties are corrected by measuring the potential at the photoconductor drum with a potential sensor.
However, according to the above-mentioned image forming apparatus, the surface potential sensor measurement circuit of the photoconductor drum and the bias circuit which performs an image producing process such as the development bias individually have the bias detection circuit. Further, the bias detection circuits are separately attached to different places due to constraints of an apparatus space. Therefore, variations in components constituting the detection circuit, temperature characteristics of the components, variations in temperature environment, and the like affect subtly detection characteristics and detection errors of the components, which generates variations in potential detection result and bias output control result. As a result, there is the problem that image densities differ from one another among the apparatuses, or the problem that difference in image density is generated according to temperature change among the apparatuses even if the image densities agree with one another under a certain condition.
Even in the same apparatus, there is the problem that the image density fluctuates according to the temperature change in the apparatus. In the case of the color image forming apparatus, there is the problem that color tint of the image is changed.
Because the temperature change in the apparatus is largely generated during continuous print in which plural sheets are printed, there is the problem that the initial print sheet differs from the print sheet, which is printed after a certain time elapses, in the image density and the initial color tint during continuous printing.
A surface temperature of the photoconductor drum varies during continuous printing, which changes a surface potential VL (light section potential) of the photoconductor drum in the maximum exposure. Therefore, there is generated the problem that the image density and the color tint are changed.
The temperature change in a bias measurement system in a primary grid changes a dark section potential VD and the light section potential VL, which generates the problem that the image density and the color tint are fluctuated.
When the light section potential VL is measured during the continuous print, sometimes there is the problem that a fog image is generated in the measurement to shorten a life of the cleaning device of the photoconductor drum.
Because the above problems are generated in each photoconductor drum, the same problems including the difference in color tint exist with respect to the fluctuation in image quality.
In the A/D conversion of the potential measurement detection result, or in the bias output detection result and the A/D conversion during the digital control of the bias circuit, since each circuit has a quantization error, and sometimes a mutual shift caused by the quantization error emerges by adding the mutual shift to a measurement error, which generates the problem that the image density is further changed.
According to the method disclosed in Japanese Patent Application Laid-Open No. H08-201461, the measurement accuracy can be increased based on the development bias output by utilizing the development bias generation device which is of the bias generating means for applying the reference voltage. However, in the case where the development bias output itself is changed due to the temperature change, there is the problem that a relationship between a charged potential and a development potential cannot be kept constant. Although the problem can be solved by repeating correction control, it is necessary that the photoconductor drum is in the floating state. Therefore, because it is necessary to stop the image forming process, the correction cannot be realized without interrupting the printing during the continuous print.